536 



TRANSACTIONS OF SECTION A. 



together and the beam of light passes obliquely across the joint. The angle 

 of the glass is obtained from a rather complicated equation by trial and error, 

 and found to be 73° 48''6 (Uviol glass). The efficiency of this form is a little 

 greater than that of a Fresnel's rhomb, and, moreover, its length is 7'5 cm. 



Since the component polarised in the plane of incidence is more copiously 

 reflected on crossing the joint than the perpendicular component is, it is neces- 

 sary to adjust the azimuth of the incident vibration to compensate for this. 

 The correct azimuth with respect to the line AB is fl where 



i 



tan n - 



cos 2 ^ — 1^) 



where <p is the angle of incidence of the beam on the glass-air surface and ty the 

 corresponding angle of refraction. For n D, = l"5035,fl =49° 2'. 



In the British Association Reports for 1851 Professor Stokes described a new 

 elliptic analyser consisting of a plate of selenite which 

 retarded waves of mean refrangibility by about a 

 quarter of a wave-length. Some experience is 

 required in its use, since we are to work by intensity, 

 and the tint produced perplexes the experimenter. 

 Still, Professor Stokes claimed very good results to be 

 A \l obtainable by his apparatus. As it is difficult to make 



/ ^J^~] p accurate settings of a quarter-wave plate and analysing 



Nicol, I thought it probable that the second form of 

 polariser, mounted ' so as to be capable of accurate 

 setting, may prove useful in the analysis of elliptic 

 vibrations. 



On a New Form of White Light Half-shade. 



The arrangement of the rhombs in the first form, 

 described above, may be used to form a white light 

 half-plate. Taking MD' = 1-5173, and the angle of 

 each rhomb 55° 15'"5, the total phase difference between 

 the components on emergence is ir. Half the field 

 Fig. 3. or view is occupied by the aperture of one of these 



rhombs and the other half by the end of one of the 

 glass blocks, C, D. These blocks partly compensate for absorption, but their 

 chief use is to compensate for the loss of light at the four reflections at normal 

 incidence. This half-shade — using white light — has been compared with the 

 Laurent half -shade — using monochromatic light— and was found quite as 

 efficient as the Laurent. Since the cost of large blocks of calcite is becoming- 

 more and more prohibitive to their use, the present form of white light half- 

 shade, which can readily be made by mounting the rhombs as indicated, may 

 prove useful. The length for an aperture of 16 cm. is about 3 cm. 



Joint Discussion with Section G on the Principles of Mechanical Flight. 

 Opened by Professor G. H. Bryan, F.R.S. 



TUESDAY, SEPTEMBER 6. 



Discussion on Atmospheric Electricity. 

 Chree, F.R.S. 



Opened by Dr. Charles 



Atmospheric electricity includes a great variety of phenomena. Omitting 

 Aurora as a subject so large as to require a separate discussion, we may mention 

 the potential gradient in the atmosphere, the influence of potential gradient 

 and electrical charges on the growth of vegetation, the phenomena of thunder- 

 storms, the loss of charge experienced by insulated bodies, the number and 



1 The edge of the glass which replaces an axis of the quarter-wave plate should 

 be specially worked. 



